Jason Aldean was in the middle of his performance at an outdoor country music festival when a gunman perched on the 32nd floor of a the Mandalay Bay Hotel and Casino on the Las Vegas Strip unleashed a shower of bullets down onto concert-goers. The artist called the incident "beyond horrific" on Instagram.

Aldean said, "I still dont know what to say but wanted to let everyone know that Me and my Crew are safe. My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night. #heartbroken#stopthehate."

After the gunman opened fire, police and SWAT teams quickly descended on the concert and the hotel-casino, and officers used explosives to get into the hotel room where the suspect was inside, authorities said.

The gunman was found dead at the scene. He was identified by Clark County Sheriff Joseph Lombardo as Stephen Paddock, 64. Investigators are still trying to discern Paddock's motive.

Police said Paddock was found dead with as many as 10 firearms in the hotel room.

The shooting at the sold-out Route 91 Harvest festival is now the deadliest mass shooting in modern U.S. history. Forty-nine people were killed when a gunman opened fire at the Pulse nightclub in Orlando in June 2016.

Sunday's shooting came more than four months after a suicide bombing at an Ariana Grande concert in Manchester, England, that killed 22 people. Almost 90 people were killed by gunmen linked to ISIS at the Bataclan concert hall in Paris during a performance by Eagles of Death Metal in November 2015.